There are a number of reasons for modifying the form of the EDM cutting pulse in such manner that the pulse would force the machining gap to ionize at the beginning of the machining power pulse and in a predictable fashion. It also is advantageous to turn on the machining power pulse or cutting current pulse in such a manner as to control the slope of the rising edge of the current pulse. This selectively variable control of the machining power pulse can operate to reduce the electrode wear, increase cutting speed and improve surface finish on the workpiece.
The prior art has recognized advantages in superimposed voltage cutting for EDM. U.S. Pat. No. 3,229,159 issued to Robert S. Webb on Jan. 11, 1966 shows one form of superimposed cutting voltage circuit. The disclosed power supply shows several arrangements of transformers with dual outputs from dual secondaries for providing the superimposed voltage operation. The power supply disclosed in that patent employs vacuum tubes for providing the switching operation.
A second and more recent U.S. Pat. No. 3,390,246 issued to Robert S. Webb on June 25, 1968 discloses an additional type of superimposed voltage circuit for EDM. The disclosure relates however to the use of a pair of electronic switches and a common triggering network again operating through a pulse transformer.
The present invention employs metal oxide, semiconductor field effect transistors, hereinafter sometimes referred to as MOSFETS, that have voltage and current ratings that permit the design of a power module and circuit that will provide a unique cutting pulse. A high voltage pulse of short duration, i.e. a voltage spike, is generated at the beginning of the main pulse thus reducing electrode wear. In addition, coupled with the high voltage spike, the current will be allowed to build up in a controlled manner and the slope of the current rise can be controlled to further reduce electrode wear. This technique of cutting further will increase cutting efficiency and surface finish since the current pulse will stimulate the spark to occur.